In present manufacture of the copper electrodes used for the foregoing purpose, hand finishing is to a large extent employed. The electrode shape is marked out on a copper or copper-tungsten block of the appropriate thickness, unwanted metal being cut away to provide a rough blank ready for finishing, which is largely a filing operation. Although some machine finishing is employed this is necessarily limited by the necessity for clamping the blank on to the machine table, and the presence of the clamps not only complicates such machining as can be performed but also precludes total machining.